Push-pull linear motor

ABSTRACT

An electro-hydraulic servo valve means comprising a four-way spool-type valve and a push-pull type linear motor for operating an axially shiftable valve member, said motor comprising a single armature arranged between a pair of spaced, axially aligned, opposing, energized field coils whereby the armature is magnetically biased at all times and subject to instant response to changes in variations in the flux field of said coils created by differentials in signal currents to said fields; said armature having shaped magnetically saturable portions related to each coil whereby magnetic saturation is effected proportionately with respect to linear motion and current differentials; said motor including equal and oppositely disposed spring means at the opposite ends of the armature yieldingly maintaining the armature in a mean position between the coils.

United States Patent Gray [ PUSH-PULL LINEAR MOTOR [76] Inventor: SamuelA. Gray, 10201 Falun Dr.,

Sun Valley, Los Angeles, Calif. 91352 22 Filed: Aug. 14, 1972 21 Appl.No.: 280,405

[56] References Cited UNITED STATES PATENTS 11/1970 Fagerlie 137/625.653/1971 Sturman 251/129 X Primary Examiner-Henry T. Klinksiek AssistantExaminer-Robert J. Miller AttorneyGeorges A. Maxwell NOV. 27, 1973 [5 7]ABSTRACT An electro-hydraulic servo valve means comprising a four-wayspool-type valve and a push-pull type linear motor for operating anaxially shiftable valve member, said motor comprising a single armaturearranged between a pair of spaced, axially aligned, opposing, energizedfield coils whereby the armature is magnetically biased at all times andsubject to instant response to changes in variations in the flux fieldof said coils created by differentials in signal currents to saidfields; said armature having shaped magnetically saturable portionsrelated to each coil whereby magnetic saturation is effectedproportionately with respect to linear motion and current differentials;said motor including equal and oppositely disposed spring means at theopposite ends of the armature yieldingly maintaining the armature in amean position between the coils.

10 Claims, 7 Drawing Figures /6 I4 13 L5 V PUSH-PULL LINEAR MOTOR Thisinvention has to do with servo-mechanisms and is more particularlyconcerned with a novel electromagnetically operated fluid valve for usein hydraulic control systems and the like.

Throughout the arts,-hydraulic motors, actuators and like devices arewidely used to move and/or effect the operation of related mechanicalmechanisms and devices and to effect work which is to be performed. Theoperation of such hydraulic devices is effected by selected andcontrolled flow of fluid, under pressure, to and from said devices bysuitable control valve structures.

While in some instances the control valve means for hydraulic devicescan be manually operated, it is often necessary and required that suchvalve means be capable of being operated or controlled by means separateand oftentimes remote from the valve means. To make possible suchoperation of control valves, it is common practice to provide the valvestructures with electrically operated drive means within closed servoloops. Such means have included solenoids, torque motors, moving coilmotors and the like. Such electro mechanically controlled valvestructures are commonly referred to and classified as electro-hydraulicservo valves.

The valve structures in most electro-hydraulic servo valves provided bythe prior art are costly and complex structures and are such that inorder to drive the valving means with those high forces required foraccurate and dependable operation, hydraulic amplification is employed.Such amplification requires complicated and delicate structures such aselectrical force motors driving flappers relative to nozzle means orsuch motors deflecting jet pipes relative to receivers.

One of the most common and widely used class of valve structures foundin the art of electro-hydraulic servo mechanisms are four-way spool-typevalves. Such spool-type valves have an elongate, axially shiftablespool-like valve member arranged in a cylindrical bore in a suitablychambered and ported fluid conducting body, with which the various fluidsupply delivery and return lines are connected.

The electric drive means provided for the last noted class and type ofvalve is generally a push-pull type of electro mechanical drive meansconnected or coupled with the valve member.

One form of push-pull type of electro mechanical drive means provided tooperate servo control valves of the character referred to above and incombination with suitable amplifying means comprises an elongatearmature with one end coupled with the valve member and spring meansnormally yieldingly holding the armature and the valve member in acentral, normal, closed position and a field coil, with a permanentmagnet, polarizing core related to and about the armature. The fieldcoil is adapted to receive control signal currents of one polarity orthe other whereby a flux field is generated which moves the armature andthe valve member axially, against the resistance of the spring means, asdesired.

By varing the strength of the signal current, the force of the field isvaried and the extent to which the armature and its related valve memberis moved by the field against the resistance of the spring means, isvaried to effect metering of the fluid flowing therethrough.

While the above form and/or class of push-pull type electro mechanicaldrive means or motors for servo valve mechanisms appears sound inprinciple and while it might be adequate and serviceable in somesituations, they have been found to be wanting and in-adequate whereimmediate or high response to control signals is required and wheregreat accuracy in metering capability is demanded.

It is a general object and feature of the present invention to providean electro-hydraulic servo valve means which requires no hydraulicamplification but which falls within that class of such valve meanswhich can only employ and require the provision and use of hydraulicamplification means.

Another object of this invention is to provide an electro magneticallyoperated servo valve including a pushpull type linear motor drive meanswhich exerts a high spool valve operating force on the order of poundsand having a natural frequency of 500 Hertz to yariatioifi 1n the inputsignal to the motor without the required use or reliance upon hydraulicamplification.

It is a further object of my invention to provide a means or structureof thecharacter referred to wherein movement of the motor armatureandresulting movement of a relative valve member is substantially linearwith respect to changes in an input signal throughout the full operatingrange of the means or structure.

It is an object and feature of my invention to provide a push-pull typelinear motor for operating an axially shiftable valve member in which asingle armature is arranged between a pair of spaced, axially aligned,opposing, energized field coils, whereby the arrnature is magneticallybiased at all times and subject to instant response to changes invariations in the flux fields of said coils and a motor includingopposing spring means at the opposite ends of the armature which serveto normally yieldingly urge and maintain the armature in a mean positionbetween the field coils.

It is another object and feature of the present invention to provide amotor of the character referred to having an armature of novelconfiguration and design whereby portions thereof become magneticallysaturated in uninterrupted progression as the armature moves toward oneor the other of the field coils in response to increases and decreasesin the magnetic fields generated, whereby the force exerted in andthrough the armature is substantially linear and the displacement ormovement of the armature is likewise substantially linear.

It is an object and feature of the instant invention to provide a meansand structure of the character referred to above which is easy andeconomical to manufacture, assemble and maintain, a structure and meanswhich is rugged, durable and both highly effective and dependable inoperation.

The foregoing and other objects and features of the present inventionwill be apparent and understood from the following detailed descriptionof a typical preferred form and carrying out of the invention throughoutwhich description reference is made to the accompanying drawings, inwhich:

FIG. 1 is a side view of myq'rnvention with portions broken away andshown in sections to illustrate certain details of the construction;

FIG. 2 is an enlarged detailed sectional view of a portion of thestructure shown in FIG. 1 and taken as indicated by line 2-2 on FIG. 1;

FIG. 3 is a detailed view taken as indicated by line 3-3 on FIG. 2;

FIG. 4 is an enlarged detailed sectional view of a portion of thestructure shown in FIG. 2;

FIG. 5 is an enlarged detailed sectional view taken substantially asindicated by line 55 on FIG. 1;

FIG. 6 is curves illustrating certain operating characteristics; and

FIG. 7 is a chart illustrating certain other operating characteristics.

Referring to FIG. 1 of the drawings, the servo valve structure providedincludes a valve means V and drive means D for operating the valvemeans.

The valve means V is shown as a four way fluid metering spool type valveincluding an elongate, axially shiftable valving member 10 and the drivemeans D is a push-pull type electro-magnetic, linear motor structureadjacent to the valve means in axial alignment with the valve member 10and coupled with one end of said valve member.

The valve means V, in addition to the valve member 10, includes anelongate body 11 with front and rear ends, a central longitudinalopening 12 with five axially spaced, annular, radially inwardly openingchannels 13, 14, 15, 16 and 17 and four fluid conducting passages 18,19, 20 and 21 communicating with certain of the channels and opening tothe exterior of the body to connect with suitable fluid supply, bypassand delivery lines (not shown). The means V further includes an elongatetubular sleeve 22 extending longitudinally in the opening 11 in thebody, closing the inner opening sides of the channels and having axiallyand circumferentially spaced, radial ports communicating with thechannels and the interior of the sleeve. The valve member 10 iscylindrical and is slidably engaged in the sleeve 12.

The valve member 10 is provided with three radially outwardly openingannular grooves 23, 24 and 25 of considerable longitudinal extent andwhich serves to establish and close communication between the portscommunicating between the interior of the sleeve and adjacent channels,upon axial shifting of the valve member in the sleeve.

The passage 18 is a fluid inlet passage communicating with the channel13 which occurs centrally of the means V and is adapted to becommunicated with a fluid supply line extending from a suitable sourceof fluid, under pressure. The passage 19 is a fluid outlet passage whichcommunicates with the channel 13 and is connected with the channels 16and 17, which occur at the opposite end portions of the body, by meansof a longitudinal tunnel 26 in the body.

The ports in the sleeve communicating with the channels 13, 16 and 17normally establish communication with the grooves 23, 24 and 25 in thevalve member and so that the channels are filled with fluid at all timesand in such a manner that the valve member is hydraulically balancedwithin the sleeve and is not subject to being urged axially therein bythe fluid pressures to which it is subjected.

The channels 14 and occur between the channels 13 and 24 and thechannels 13 and 25, respectively, and the ports in the sleevecommunicating therewith are normally closed by the portions of the valvemember 10 at the opposite ends of the central groove 23 therein. Thepassages 20 and 21 communicate with the channels 14 and 15 and areadapted to connect with fluid conducting lines extending to relatedfluid openings in a hydraulically operated motor, actuator or otherdevice with which my construction is related to control the operationthereof.

Upon shifting the valve member 10 forwardly, to the right in FIG. 1 ofthe drawings, the port to the channel 15 is open to the groove 23 andfluid is therefor conducted through the passage 21 and to the relatedhydraulic device. The port to channel '18 is open to groove 24 and fluidfrom the device is free to flow back into the valve, through the groove24, channel 16 and out through the passage 13. It will be apparent thatupon shifting of the valve member rearwardly from its normal or closedposition, as shown, a reversed flow pattern from that which is describedabove is effected.

It is to be noted and it is important to understand that metering andcontrolling the volume of fluid conducted to and from the device withwhich the valve means is related is effected by controlling the extentof axial movement of the valve member and the extent to which the portscommunicating with the channels 14 and 15 are opened.

The valve structure described above is a typical fourway fluid meteringspool type valve structure and its construction and mode of operation iswell known to those skilled in the art.

In the case illustrated, the front end of the valve body is closed by acap 26 screw threaded into a socket opening in that end of the body andso that free access to the interior of the body and to the valve member,for purposes of assembly, service and/or adjustment can be readily had.

In the form of the invention illustrated, the valve member 10 has acentral, longitudinal bore to freely accommodate an operating rod 27extending from the drive means D. The front end of the rod is coupledwith the front end of the member 10 in axial driving relationship bymeans of a coupler 28 threaded into the front end of the bore in themember 10 and into and through which the front end of the rod isthreaded. The coupler 28 is accessible in the socket in the front end ofthe body and is so arranged that coupling of the member and rod andaxial adjustment thereof can be easily made. The body can, as shown, beprovided with suitable mounting means, such as apertured flanges throughwhich mounting screw fasteners can be engaged.

The rear end of the body 11 is provided with an enlarged, rearwardlyopening bell-shaped housing portion 30 with an annular, rearwardlydisposed mounting shoulder 31 to accommodate a portion of the means Dand in which the means D is secured or mounted. The rear end of thehousing portion 30 is internally threaded as at 32 and the forward endof an elongate, forwardly opening cup-like closure 33 is engagedtherein. The closure 33 freely surrounds and protects that portion ofthe means D which projects rearwardly from the portion 30 of the body11, as clearly illustrated in the drawings.

The drive means D, as noted above, is a push-pull type electro-magnetic,linear motor structure.

The means D, while particularly designed and constructed for operatingthe valve means V here provided, or a valve means of similar nature, itis such that it can be advantageously used or employed in many othersituations and to operate other means or devices where push-pull typeprime mover or drive means are required.

In the form of the invention illustrated, the means D includes anelongate, cylindrical, tubular support barrel 40 with front and rearopen ends and having a radially outwardly projecting mounting flange 41at its front end. The flange 41 engages and stops against the shoulder31 in the bell portion 30 of the body 11 and is secured thereto byfasteners 32.

The barrel 40 supports and carries a pair of like, oppositely disposedelectro magnet units, there being a front unit M and a rear unit M.

Each of the units M and M include an elongate cyylindrical core 45 infixed position in an end portion of the barrel and having an outer enddisposed axially outwardly from its related end portion of the barreland a flat, radially extending, axially inwardly disposed inner end.Each core is further characterized by a central longitudinal bore 46with an enlarged socket 47 opening at its outer end and an annular,axially inwardly opening coil recess 48 entering the inner end anddefining inner and outer annular pole faces N and S at said inner end ofthe core.

The coil recess in each core has a cylindrical inner and outer walls anda flat bottom and cooperatively receives and holds a field winding W.

Suitable openings are provided in the outer end of each core toaccommodate the leads or conductors for the winding W, as illustrated at49.

In addition to the above, each core 45 is provided with a plurality (3)of circumferentially spaced threaded studs 50projecting axiallyoutwardly from the outer end of the core. The studs 50 are shownthreadedly engaged in suitable openings provided in the core.

It is important to note that the wall thickness of the core occurringradially outward of the socket 45 is less than the wall thickness of thecore occurring radially inward of the socket 45 whereby thecross-sectional areas of the core, radially inward and outward of thesocket 45 are equal and the areas of the pole faces N and S are equal.

The structure provided next includes an elongate armature shaft 51extending longitudinally and freely through the bores 46 in the cores ofthe units M and M and bridging the space between said units.

The ends of the shaft 51 terminate in the socket openings 47 in theouter ends of the cores 45 and are threaded.

The shaft 51 carries an armature A intermediate its ends which armatureis a 'flat, radially extending, discshaped part and projects radially,freely and in spaced relationship between the pole faces N and S of theunits M and M.

The special form and nature of the armature will be fully described andconsidered later in this disclosure.

The structure next includes axially shiftable support means B to supportand maintain the shaft 51 and the armature A concentric in and with theunits M and M.

The means B includes two, like, support springs 52 coupled with theopposite ends of the shaft 51 and with the outer end of the units M andM' related thereto. The springs 52 have central hub portions 53 and aplurality (3)' of circumferentially spaced, radially outwardlyprojecting, substantially U-shaped spring arms 54 with aperturedterminal ends engaged on and about the studs 50 of their related units Mand M, as clearly illustrated in FIGS. 2 and 3 of the drawings. Thesprings 52 are spaced from the cores of their related units M and M bywashers 53 and are secured in fixed relationship by holding units 54 onthe studs outward of the springs.

The hub portions 53 of the springs 52 are provided with tubular axialtubes 54 which project axially inwardly into the sockets 47 in the cores45 of the units M and M and which threadedly receive and couple with theends of the shaft 51 related thereto.

The threaded connections between the springs 52 and ends of the shaft 51permit for axial shifting and adjusting of the shaft and the armature Athereon. Such axial adjustment can be effected by engaging and rotatingthe armature through an access opening 53 provided in the centralportion of the sleeve 40, as shown in FIG. 1 and FIG. 2 of the drawings.

The springs 52 are flat, normally radially extending parts and arenormally unbiased. When in their normal position, the central radialplane of the armature A on the shaft is midway between the opposing polefaces of the units M and M and the opposite, flat, radially extendingaxially disposed surfaces of the armature A are in equal predeterminedspaced relationship from said pole faces.

The axial parts of the springs are further provided with axiallyoutwardly projecting bearings 55, in the nature of semi-sphericalprotuberances.

The structure next includes spring-loading means L and L at the frontand rear ends of the means D which loading means are similar in natureand serve to exert equal and opposite, axially directed forces onto andthrough the armature shaft 51.

The means L', at the rear end of the means D, includes an axiallyinwardly opening cup 60 with a cylindrical side wall 61, a flat bottom62 at its outer end and a radially outwardly projecting mounting flange63 at its inner end. The flange is apertured to receive the outer endportions of the studs 50 on the unit M and is secured in fixedrelationship on or with the unit M by lock nuts 64 on the studs 50,outward of the flange and holding the flange tight against the nuts 54on the studs.

The means L' next includes a substantial disc-shaped radially extendingspring seat 66 with a central bearing socket 67, axially outward of thespring 52 and in which the bearing 55 related to the rear end of theshaft 51 is seated.

The means L' next includes an elongate, axially extending helicalcompression load spring 67 within the cup with a front end seated on theseat 66 and axially adjustable setting means D carried by the bottom 62of the cup and engaging the rear end of the spring 67. The means Dincludes an internally threaded tubular stem 68 fixed in a centralopening 69 in the bottom 62 of the cup 60, an axially adjustable settingscrew 70 in the stem 68, to project axially forwardly therefrom and afollower 71 engaged by the screw 70 and engaged with the rear end of thespring 67. The follower 71 is an elongate, rearwardly opening, cuplikepart which extends axially into the spring 67 and occurs about the stem60 and screw 70; has a conical bottom 72 at its front end in which thescrew is seated and stopped and has a radially outwardly projecting seatflange 73 at its rear end engaged with the rear end of the spring 67.

With the means L' set forth above, it will be apparent that the extentto which the spring 67 is biased can be easily and convenientlyadjusted.

The screw 70 is shown as an Allen screw engaged in the stem 68 andaccessible from the open rear end of the stem.

The means L includes a cup 60, similar to the cup 60 of the means L,mounted and fixed relative to the front end of the unit M in the samemanner that the cup 60 of the means L is related to the means M, exceptthat the nuts 54 related to the means L are shorter than the nuts 54related to the means L. The means L includes a spring seat 66 and acompression load spring 67 identical with the seat 66 and spring 67 ofthe means L. In the means L, the means D provided in the means L is notprovided and the front end of the spring 67 sits directly on the bottom62 of the cup 60.

With the means L and L set forth above, it will be apparent that uponaxial adjustment of the means D in the means L, the springs 67 of boththe means L and L are adjusted equally and at the same time.

In practice, the springs 67 of the means L and L are compressed andbiased or preset to an axial extent greater than the maximum axialmovement of the armature A and rod 51 and act equally and oppositely atand upon the opposite ends of the rod 51 to normally yieldingly hold therod 51 and the armature A in their normal central position, as shown inFIG. 2 of the drawmgs.

The load springs 67 are of considerable strength with respect to themass of the rod 51 and armature A and are therefor such that the springrate of said springs .govems the operation of the construction asregards its natural frequency.

The operating rod 27 coupled with and extending rearwardly through thevalve member 10 of the valve means V, projects rearwardly from the valvemeans V, through the opening 69 of the bottom 62 of the cup 60 by meansL, extends through a central opening 80 provides in the spring seat 66,thence through a cental opening 81 in the bearing 55 of the means L andto or with which it is fixed. In practice the rear end of the rod isthreaded in the opening 81 and is fixed thereon as suitable stacking therearmost threads thereof.

With the structure set forth above, it will be apparent that the means Vand D are suitable coupled and housed to establish a neat, compact, easyand economical to make, assemble and maintain servo valve construction.

It will be further apparent that with and by means of the severalaxially shiftable threaded couplings and means provided, accurateadjustment of the springs and position of the armature and the valvemember can be easily and conveniently made.

The armature A, as previously noted, is a flat radially extendingdisc-shaped body of magnetic material and is characterized by flatforwardly and rearwardly disposed end faces 90 and 91, which oppose theinner sides of the units M and M and the pole surfaces N and S thereof,a cylindrical, radially outwardly disposed exterior surface 92 and aradially outwardly opening annular groove 93 in the surface 92. Thecentral plane of the groove 93 is on the central plane of the armatureand has axially and radially inwardly parabolically curved side surfaceswhich cooperate with their adjacent end portions of the armature todefine peripheral edge portions of diminishing cross-section adjacentand about the outer portion of each surface 90 and 91. The edge portionsof diminishing cross-section at and about the outer periphery of thesurfaces 90 and 91 of the armature A are so formed that as the portionof the armature with which they are related is subjected to increasedmagnetic field forces, the edge portions become saturated from theexterior and radially inwardly at a predetermined rate.

In operation, the coils or windings W of the means M and M are normallyenergized by equal, signal currents and so that the outercircumferential poles N of the, for example, units are north and theinner poles S are south. The lines of flux between the poles of theunits M and M, in the air gap, are conducted, from north to south,through the half of the armature A which opposes the poles of thoseunits and occurs at the opposite ends or sides of the central verticalplane of the armature. The quiescent currents are such that the annaturematerial is magnetically biased, that is, it is magnetized to thatextent that it will accommodate additional lines of flux freely andwithout the conducting of added or extra current to the field windingsto effect magnetizing of the armature.

It is important to note that while the armature A is normallymagnetically biased as set forth above, the motive effect on themagnetic fields of the units M and M thereon is equalized one by theother and the armature remains in its central, normal position with thesurfaces and 91 thereof spaced equal distances from their opposingrelated poles of the units M and M.

Upon an increase of current to the winding W of one unit, and a decreaseof current to the other winding, the normal balance of the fields actingon and through the armature is upset and the armature is urged towardssaid one unit, away from the other unit.

As the armature A moves towards said one unit the saturable portion ofthe armature related to that unit, that is the peripheral portion ofdiminishing crosssection at the end of the armature adjacent that unitbecomes progressively magnetically saturated in a predetermined rationbetween the force of the field of that unit and the diminishing distancebetween the armature and said one unit, and so that the effect of thatfield on and through the armature is to cause motion of the armaturewhich is linear with respect to the differential of the input currents.

The armature A and the units M and M are designed, balanced and relatedto each other so that the axial movement of the armature between theunits M and M is linear with the differential in currents supplied tothe units M and M.

With the drive means D provided, it will be apparent that the means D isenergized and in full operating condition at all times and that themechanical movement to be effected thereby, be it right or left ofcenter, occurs instantly in direct linear response to differentialsestablished in the currents to the field units M and M.

In FIG. 6, line X indicates the linear displacement of my new armaturewith respect to differentials in signal current. Line Y indicates thenature of saturation of the armature A in response to currentdifferentials and displacement of the armature. Line Z indicates theperformance characteristics of an armature in the instant drive meanswhich is not grooved or provided with the saturable portions hereprovided.

In the chart shown in FIG. 7 of the drawings, the performancecharacteristics of one embodiment of my new construction is illustrated.It is to be noted that the differentials in amperes is that difierentialwhich is brought about by current added and subtracted to the normalquiescent currents to the units M and M which quiescent currents can bewhatever current is required to effect the desired magnetic biasing ofthe unit cores and those portions of the armature related thereto.

Having described only one typical preferred form and application of myinvention, I do not wish to be limited or restricted to the specificdetails herein set forth, but wish to reserve to myself anymodifications and/or variations that may appear to those skilled in theart to which this invention pertains.

Having described my invention, 1 claim:

1. A servo valve having an elongate valving member with front and rearends and shiftable axially forwardly and rearwardly from a centralnormal position; drive means to shift the valve member forwardly andrearwardly to and from its central normal position comprising anelongate armature shaft with front and rear ends, coupling meansconnecting the front end of the shaft with the valving member, anarmature with forwardly and rearwardly, axially outwardly disposedsurfaces carried by the shaft between the ends thereof, front and rearelectro magnet field units arranged forwardly and rearwardly of thearmature, respectively, and having axially inwardly disposed north andsouth poles in normal predetermined spaced and opposing relationshipwith the front and rear surfaces of the armature related thereto,support means at the ends of the shaft and maintaining the shaft on acommon axis and spring loading means at the opposite ends of the shaftbiased to exert equal and opposing forces axially inwardly at the endsof the shaft and normally yieldingly holding the shaft with the armaturein its normal position, said field units normally energized by quiescentcurrents and establishing equal and opposite magnetic fieldsintersecting the armature whereby the armature is normally magnetizedand held in its normal position by the fields, said armature and shaftbeing shifted axially toward one field unit upon an increase of currentthereto and a decrease of current to the other field unit, resulting inincrease and decrease in the magnetic fields of the units acting on thearmature, the armature having progressively magnetically saturableportions related to each surface thereof and to the field unit relatedthereto whereby the saturable portion of the armature to a field unittowards which it is moved saturates progressively with respect to itslinear longitudinal movement and linear differentials in the currents tothe field units.

2. A structure as set forth in claim 1 wherein the field units includeelongate axially aligned, axially spaced cylindrical cores with centralopenings through which the shaft freely projects, flat axially inwardlyopposing inner ends, annular, axially inwardly opening grooves openingat said inner ends and defining axially inwardly disposed radiallyspaced annular pole faces and field coils engaged in the grooves, saidarmature being a flat, radially extending disc-shaped part on the shaftextending freely between the inner end of the field units and having anouter cylindrical side with a radially outwardly opening shaped channeldefining said magnetically saturable portions related to the surfaces atthe opposite ends thereof, and means engaged with and between the coresto maintain the units in fixed relationship with each other.

3. A structure as set forth in claim 2 wherein said support means forthe shaft includes axially yielding radially non-yielding spring unitsmounted in fixed spaced position from axially outwardly disposed ends ofthe field units and means connecting each spring unit with an end of theshaft.

4. A structure as set forth in claim 3 wherein said spring loading meansincludes spring seats, bearing support means between the seats and theends of the shaft, axially force exerting springs with inner and outerends, said inner ends engaging the spring seats and spring stopsengaging the outer ends of said springs.

5. A structure as set forth in claim 3 wherein said spring loading meansincludes spring seats, bearing support means between the seats and theends of the shaft, elongate, axially extending helical compressingsprings with inner ends engaging the spring seats and spring stopsengaging the other outer ends of the compression springs, the springstops related to one spring being in fixed position relative to thefield units and the other spring stop having a part in fixed positionrelative to the field units, an axially moveable part and axiallyshiftable screw means between the parts whereby the moveable part can bemoved axially to bias the springs.

6. A structure as set forth in claim 3 wherein the field units includeelongate axially aligned, axially spaced cylindrical cores with centralopenings through which the shaft freely projects, flat axially inwardlyopposing inner ends, annular, axially inwardly opening grooves opeing atsaid inner ends and defining axially inwardly disposed radially spacedannular pole faces and field coils engaged in the grooves, said armaturebeing flat, radially extending disc-shaped part on the shaft extendingfreely between the inner ends of the field units and having an outercylindrical side with a radially outwardly opening shaped channeldefining said magnetically saturable portions related to the surfaces atthe opposite ends thereof, and means engaged with and between the coresto maintain the units in fixed relationship with each other andincluding an elongate barrel with open ends and in which the cores areengaged and held, said barrel having a mounting flange projectingtherefrom to engage a related support structure.

7. A structure as set forth in claim 6 wherein said support means forthe shaft includes axially yielding radially non-yielding spring unitsmounted in fixed spaced position from axially outwardly disposed ends ofthe field units and means connecting each spring unit with an end of theshaft.

8. A structure as set forth in claim 7 wherein said spring loading meansincludes spring seats, bearing support means between the seats and theends of the shaft, elongate, axially extending helical compressingsprings with inner ends engaging the spring seats and spring stopsengaging the other outer ends of the compression springs, the springstops related to one spring being in fixed position relative to thefield units and the other spring stop having a part in fixed positionrelative to the field units, an axially moveable part and axiallyshiftable screw means between the parts whereby the moveable part can bemoved axially to bias the spring.

9. A structure as set forth in claim 7 wherein said spring loading meansincludes spring seats, bearing support means between the seats and theends of the shaft, elongate, axially extending helical compressingsprings with inner ends engaging the spring seats and spring stopsengaging the other outer ends of the compression springs, the springstops relate to one spring being in fixed position relative to the fieldunits and the other spring stop having a part in fixed position relativeto the field units, an axially moveable part and axially shiftable screwmeans between the parts whereby the moveable part can be moved axiallyto bias the springs, said coupling means connecting the support springsto the ends of the shaft including threaded portions on the ends of theshaft and threaded parts carried by the 12 springs, the spring stopsrelated to one spring being in fixed position relative to the fieldunits and the other spring stop having a part in fixed position relativeto the field units, an axially moveable part and axially shiftable screwmeans between the parts whereby the moveable part can be moved axiallyto bias the springs, said coupling means connecting the support springsto the ends of the shaft including threaded portions on the ends of theshaft and threaded parts carried by the spring and engaged with saidthreaded portions whereby the longitudinal position of the armature canbe adjusted by rotating the armature and shaft to advance the threadedportions in the threaded parts, said barrel having an access opening forengaging the annature to rotate said armature and shaft.

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1. A servo valve having an elongate valving member with front and rearends and shiftable axially forwardly and rearwardly from a centralnormal position; drive means to shift the valve member forwardly andrearwardly to and from its central normal position comprising anelongate armature shaft with front and rear ends, coupling meansconnecting the front end of the shaft with the valving member, anarmature with forwardly and rearwardly, axially outwardly disposedsurfaces carried by the shaft between the ends thereof, front and rearelectro magnet field units arranged forwardly and rearwardly of thearmature, respectively, and having axially inwardly disposed north andsouth poles in normal predetermined spaced and opposing relationshipwith the front and rear surfaces of the armature related thereto,support means at the ends of the shaft and maintaining the shaft on acommon axis and spring loading means at the opposite ends of the shaftbiased to exert equal and opposing forces axially inwardly at the endsof the shaft and normally yieldingly holding the shaft with the armaturein its normal position, said field units normally energized by quiescentcurrents and establishing equal and opposite magnetic fieldsintersecting the armature whereby the armature is normally magnetizedand held in its normal position by the fields, said armature and shaftbeing shifted axially toward one field unit upon an increase of currentthereto and a decrease of current to the other field unit, resulting inincrease and decrease in the magnetic fields of the units acting on thearmature, the armature having progressively magnetically saturableportions related to each surface thereof and to the field unit relatedthereto whereby the saturable portion of the armature to a field unittowards which it is moved saturates progressively with respect to itslinear longitudinal movement and linear differentials in the currents tothe field units.
 2. A structure as set forth in claim 1 wherein thefield units include elongate axially aligned, axially spaced cylindricalcores with central openings through which the shaft freely projects,flat axially inwardly opposing inner ends, annular, axially inwardlyopening grooves opening at said inner ends and defining axially inwardlydisposed radially spaced annular pole faces and field coils engaged inthe grooves, said armature being a flat, radially extending disc-shapedpart on the shaft extending freely between the inner end of the fieldunits and having an outer cylindrical side with a radially outwardlyopening shaped channel defining said magnetically saturable portionsrelated to the surfaces at the opposite ends thereof, and means engagedwith and between the cores to maintain the units in fixed relationshIpwith each other.
 3. A structure as set forth in claim 2 wherein saidsupport means for the shaft includes axially yielding radiallynon-yielding spring units mounted in fixed spaced position from axiallyoutwardly disposed ends of the field units and means connecting eachspring unit with an end of the shaft.
 4. A structure as set forth inclaim 3 wherein said spring loading means includes spring seats, bearingsupport means between the seats and the ends of the shaft, axially forceexerting springs with inner and outer ends, said inner ends engaging thespring seats and spring stops engaging the outer ends of said springs.5. A structure as set forth in claim 3 wherein said spring loading meansincludes spring seats, bearing support means between the seats and theends of the shaft, elongate, axially extending helical compressingsprings with inner ends engaging the spring seats and spring stopsengaging the other outer ends of the compression springs, the springstops related to one spring being in fixed position relative to thefield units and the other spring stop having a part in fixed positionrelative to the field units, an axially moveable part and axiallyshiftable screw means between the parts whereby the moveable part can bemoved axially to bias the springs.
 6. A structure as set forth in claim3 wherein the field units include elongate axially aligned, axiallyspaced cylindrical cores with central openings through which the shaftfreely projects, flat axially inwardly opposing inner ends, annular,axially inwardly opening grooves opeing at said inner ends and definingaxially inwardly disposed radially spaced annular pole faces and fieldcoils engaged in the grooves, said armature being flat, radiallyextending disc-shaped part on the shaft extending freely between theinner ends of the field units and having an outer cylindrical side witha radially outwardly opening shaped channel defining said magneticallysaturable portions related to the surfaces at the opposite ends thereof,and means engaged with and between the cores to maintain the units infixed relationship with each other and including an elongate barrel withopen ends and in which the cores are engaged and held, said barrelhaving a mounting flange projecting therefrom to engage a relatedsupport structure.
 7. A structure as set forth in claim 6 wherein saidsupport means for the shaft includes axially yielding radiallynon-yielding spring units mounted in fixed spaced position from axiallyoutwardly disposed ends of the field units and means connecting eachspring unit with an end of the shaft.
 8. A structure as set forth inclaim 7 wherein said spring loading means includes spring seats, bearingsupport means between the seats and the ends of the shaft, elongate,axially extending helical compressing springs with inner ends engagingthe spring seats and spring stops engaging the other outer ends of thecompression springs, the spring stops related to one spring being infixed position relative to the field units and the other spring stophaving a part in fixed position relative to the field units, an axiallymoveable part and axially shiftable screw means between the partswhereby the moveable part can be moved axially to bias the spring.
 9. Astructure as set forth in claim 7 wherein said spring loading meansincludes spring seats, bearing support means between the seats and theends of the shaft, elongate, axially extending helical compressingsprings with inner ends engaging the spring seats and spring stopsengaging the other outer ends of the compression springs, the springstops relate to one spring being in fixed position relative to the fieldunits and the other spring stop having a part in fixed position relativeto the field units, an axially moveable part and axially shiftable screwmeans between the parts whereby the moveable part can be moved axiallyto bias the springs, said coupling means connecting the support springsto the ends of the shaft including threaDed portions on the ends of theshaft and threaded parts carried by the spring and engaged with saidthreaded portions whereby the longitudinal position of the armature canbe adjusted by rotating the armature and shaft to advance the threadedportions in the threaded parts.
 10. A structure as set forth in claim 7wherein said spring loading means includes spring seats, bearing supportmeans between the seats and the ends of the shaft, elongate, axiallyextending helical compressing springs with inner ends engaging thespring seats and spring stops engaging the other outer ends of thecompression springs, the spring stops related to one spring being infixed position relative to the field units and the other spring stophaving a part in fixed position relative to the field units, an axiallymoveable part and axially shiftable screw means between the partswhereby the moveable part can be moved axially to bias the springs, saidcoupling means connecting the support springs to the ends of the shaftincluding threaded portions on the ends of the shaft and threaded partscarried by the spring and engaged with said threaded portions wherebythe longitudinal position of the armature can be adjusted by rotatingthe armature and shaft to advance the threaded portions in the threadedparts, said barrel having an access opening for engaging the armature torotate said armature and shaft.